1. Field of the Invention
The present invention relates to an image forming device such as a copying machine, a printer, a facsimile machine, a multifunctional machine having functions of two or more of these and the like, and further to a method for controlling a power supply for transfer for transferring a toner image in such an image forming device onto a recording sheet such as a recording paper sheet.
2. Description of Related Art
Today, various types of image forming devices are known. Among them, there are such image forming devices which can form a toner image on an image carrier whose surface moves, and transfer the toner image onto a recording paper sheet, a recording sheet for an overhead projector or the like (hereinafter these may be collectively referred to as “recording paper sheet” or simply “paper sheet”) by a rotatable transfer member which is pressed against the image carrier and to which a high voltage for transfer is applied from a power supply for transfer, the recording paper sheet being fed into a pressing nip between the image carrier and the transfer member.
Herein, common examples of the image carrier on which the toner image is formed and from which the toner image is to be transferred onto the recording paper sheet by the transfer member, are electrostatic latent image carriers such as a photosensitive member in electrophotographic monochrome image forming device.
In addition, for example, in an image forming device which comprises a plurality of electrophotographic image forming units which can form an electrostatic latent image on an electrostatic latent image carrier and develop the electrostatic latent image to form a toner image (including when a part of each image forming unit is shared by a plurality of the image forming units), in which the toner images formed in the image forming units are primarily transferred onto an intermediate transfer member such as an intermediate transfer belt, and the toner images on the intermediate transfer member are secondarily transferred onto a recording paper sheet by a secondary transfer member, the intermediate transfer member corresponds to the above-mentioned “the image carrier from which the toner image is to be transferred onto the recording paper sheet by the transfer member”, and the secondary transfer member corresponds to the transfer member.
In either case, in an image forming device which can form a toner image on an image carrier whose surface moves, and transfer the toner image onto a recording paper sheet by a rotatable transfer member which is pressed against the image carrier and to which a high voltage for transfer is applied from a power supply for transfer, the recording paper sheet being fed into a pressing nip between the image carrier and the transfer member, in general, the recording paper sheet is fed from a recording paper sheet feed part; paused by a timing roller unit; and fed into the pressing nip between the image carrier and the transfer member by the timing roller unit at a timing when the toner image on the image carrier which reaches the pressing nip can be transferred onto the recording paper sheet.
As for the transfer member, if it is pressed against the image carrier at all times, it may be deformed due to creep deformation or other causes when the image forming device is not used for a long period of time or in other situations, and the deformation may lead to deteriorated image formation. Therefore, in some cases, the transfer member employed is provided to be capable of coming into contact with and retreating from the image carrier by a contact-retreat drive unit, and is pressed against the image carrier from the state of being away from the image carrier by an instruction of a controller of the contact-retreat drive unit, prior to the initiation of transfer of the toner image onto the recording paper sheet by the transfer member.
In some cases, the power supply for transfer is such a power supply which is turned on by a power supply ON instruction signal from a power supply controller, being turned off by a power supply OFF instruction signal, and which uses the voltage set by an output voltage setting signal from the power supply controller as an output voltage when the power supply is on so that a high voltage for transfer applied to the transfer member can be selected from a wide range from the minus side to the plus side. In this case, the power supply ON instruction signal from the power supply controller is also referred to as “remote signal”, and the power supply is turned on by the remote signal ON.
An example of such a power supply for transfer include a power supply comprising a first high voltage generating part which generates a voltage of a first polarity (for example, negative polarity) and a second high voltage generating part which generates a voltage of a second polarity (for example, positive polarity) which is different from the first polarity, and which is turned on by a power supply ON instruction signal from a power supply controller, being turned off by a power supply OFF instruction signal from the power supply controller, and which uses the voltage set by an output voltage setting signal from the power supply controller as an output voltage when the power supply is on.
In the image forming device in which the transfer member which is capable of pressing against, coming into contact with and retreating from the image carrier by the contact-retreat drive unit is employed, in pressing the transfer member for transferring the toner image on the image carrier onto the recording paper sheet against the image carrier, when the transfer member is pressed against the image carrier in a state that a high voltage for transfer is applied to the transfer member, unallowable discharge may occur between the transfer member and the image carrier, leading to the phenomenon known as memory causing an image noise on the image carrier.
Accordingly, in general, when there is a request for image formation, the transfer member is pressed against the image carrier first, and then the high voltage for transfer is applied to the transfer member.
When a power supply employed is the above-mentioned power supply, i.e., a power supply such that is turned on and off by the remote signals ON and OFF, and that uses the voltage set by the output voltage setting signal as an output voltage when it is on, the transfer member is pressed against the image carrier, and then the output voltage is set to the high voltage for transfer by the output voltage setting signal with the remote signal ON.
Meanwhile, JP2006-208497A describes that a toner image is secondarily transferred onto the recording paper sheet by a secondary transfer roller, which is the transfer member, from an intermediate transfer belt, which is an image carrier holding the toner image to be transferred onto the recording paper sheet.
In addition, the document describes that after the secondary transfer roller is pressed against the intermediate transfer belt, an entrance voltage lower than a defined high voltage for transfer by a predetermined value is applied to the secondary transfer roller from the time when the front end of the recording paper sheet enters into the pressing nip between the roller and the belt until a predetermined period elapses, and the defined high voltage for transfer is then applied so that the voltage fluctuation of the secondary transfer roller at the entrance of the paper sheet is suppressed and images can be successfully transferred onto the front end portion of the paper sheet.
However, in an image forming device comprising
an image carrier which is a rotatable image carrier, is capable of forming a toner image on a circumferential surface thereof and is capable of rotating while holding the toner image thereon,
a transfer member which is for transferring the toner image on the image carrier onto a recording paper sheet and capable of rotating, provided in a manner capable of pressing against, coming into contact with and retreating from the image carrier, and which forms a pressing nip by being pressed against the image carrier,
a power supply for transfer for applying a voltage to the transfer member for transfer for transferring the toner image on the image carrier onto the recording paper sheet, and
a recording paper sheet feed device for feeding the recording paper sheet into the pressing nip,
the power supply for transfer being turned on by a power supply ON instruction signal, being turned off by a power supply OFF instruction signal, and using a voltage set by an output voltage setting signal as an output voltage when it is on, after the transfer member is pressed against the image carrier, when the output voltage is set to a high voltage for transfer by the output voltage setting signal upon turning on the power supply by the power supply ON instruction signal, the rise of the output voltage of the power supply to the high voltage for transfer may be delayed.
As a result, if the distance from the front end of the recording paper sheet which is being paused normally by the timing roller unit to the pressing nip between the image carrier and the transfer member is set shorter to achieve size reduction and compactness of the image forming device and for other reasons, the rise of the high voltage for transfer is delayed with respect to the entrance of the recording paper sheet into the pressing nip, and considerable transfer failure may occur at an early stage of the transfer of the toner image onto the recording paper sheet.
The JP2006-208497A does not refer to this matter.